Apparatus and methods for generating and interrogating holograms and other optical diffractive devices have been known since the early 1960's however such devices were largely sophisticated laboratory research tools. More recently, the advent of high resolution color photocopying, digital scanning, and image processing have made counterfeiting documents incorporating only 2-dimensional information much simpler. Holograms and other media capable of storing 3-dimensional information have become increasingly important in preventing counterfeiting and assuring the authenticity of documents and items attached to such documents. For example, it is common for Visa and MasterCard type credit cards to include an embossed "rainbow" hologram on the front of the card. Computer software developers including Lotus Development Corporation, Microsoft Corporation, and others have included a holographic label on their product packaging to help distinguish their product from a potential unauthorized reproduction of their product. It is also anticipated that commercial instruments, including stock certificates, currency, and other negotiable financial instruments, may someday benefit from the inclusion of holographic indicia on the instrument itself.
The incorporation of holographic security devices, such as the holograms built into credit cards and fused to negotiable instruments, as well as applications to currency, raises a need for high quality holograms that cannot easily be counterfeited and that can be produced quickly and cheaply in volume to reduce the cost per hologram or diffraction grating. High resolution holographic grating array masters are currently produced using E-Beam techniques and are therefore relatively expensive and slow to manufacture. Other methods of producing lower resolution grating arrays have become commonplace enough to threaten their potential usefulness in the field of security. Therefore, there is a need for high resolution but inexpensive gratings that cannot easily be produced by counterfeiting.
Holographic diffraction grating arrays have been manufactured since as early as 1980. In the last few years, high resolution grating arrays have found a new use in the security field as anti-counterfeiting devices. The technologies used in the manufacture of these arrays include holographic recording of spot diffraction gratings and E-Beam etching of the holographic grating fringe structure itself.
Conventional holographic systems extant at this time seem to be limited to spot sizes and therefore feature sizes on the order of 1/400+L inch (about 60 microns). Even at this relatively large feature size, the recording of large array areas with conventional techniques can be prohibitively slow (and therefore expensive) since each image pixel typically requires individual motion of the film platen and subsequent exposure. These conventional systems are also generally limited to fixed spatial frequencies, or at best to a predetermined set of fixed spatial frequencies, which limits the type of imagery or graphical information they are capable of recording. There are also quite a number of such systems in operation at this time, a situation which seriously limits their credibility and utility in the field of security.
E-Beam writing of grating arrays which is capable of high resolution, is so prohibitively expensive a process and requires such a large capital investment that it's utility in anti-counterfeiting is secure. It is not seriously limited as to feature size as each fringe of any desired grating array can be written individually. Unfortunately, the production of relatively small grating arrays requires a considerable amount of time on extremely expensive equipment. As with the invention described herein, clever programming is also necessary to take full advantage of such as system. E-Beam recording techniques also require relatively sophisticated equipment control and computer control techniques to achieve the desired output.
To date there seems to be no system extant capable of producing inexpensive, large area, high resolution, grating arrays. Therefore, it is clear that there is a need for system, apparatus, and method that provide solutions to these limitations in the prior art.
In one aspect the invention provides means for controlling the shape of the exposure area in a very accurate and precise manner, and to generate final images that contain microscopic two-dimensional image data such as text or other graphical art, where such data can be unique for each exposure area.
In another aspect the invention provides means for representing features in greater detail than provided in conventional systems.
In another aspect the invention provides means for controlling the shape of each output exposure footprint.
In yet another aspect the invention reduces pixelation, quantization, or stair stepping and the rough edges which result from pixel quantization in the final output image so that the output image follows the actual contours in the macroscopic image.
In another aspect the invention provides means for adjusting the output grating spatial frequencies and spatial orientations at will, including presenting or altering the frequencies and orientations continuously and in real-time during operation of the apparatus.
In another aspect the invention provides means for modifying or controlling the spatial frequencies during operation without changing the optical configuration.
In another aspect the invention provides means for generating a multitude of diffractive patterns rapidly, with the potential for generating the diffractive patterns in real time.
In another aspect the invention eliminates moving parts in the optical system and to thereby increase speed and provide greater stability.
In another aspect the invention provides means for more readily maintaining system alignment than would be provided in conventional two beam configurations, or configurations involving mirrors or prisms.
In another aspect the invention provides means for generating diffractive areas which are smaller than have been realized with conventional systems.
In another aspect the invention provides means for forming multi-directional gratings in a single exposure.
In another aspect the invention provides means for accurately and precisely controlling diffraction efficiency by modifying the contrast of the image displayed on the image display device.
In yet a further aspect the invention provides means for using system noise characteristics (including noise introduced by the optical system and image display device imperfections) to uniquely identify the system from which a particular hologram or grating was produced, as a security feature to verify the hologram or grating authenticity.
In another aspect the invention provides means for generating holograms having even exposure density across the entire hologram and for eliminating the Gaussian pits that are typically present in conventional systems which use a focussed narrow beam, by providing a filtered and expanded beam to illuminate the image display device.
In another aspect the invention provides a system permitting rapid photographic exposure by reducing settling times typically required in conventional systems having moving optical components and hence vibration, and by providing a large exposure footprint even for small pixel dimensions, and for providing means for exposing multiple angles in the same exposure footprint simultaneously.
These and other features and advantages are provided by the present invention as will be readily apparent in light of the accompanying drawings and detailed description.